In microwave communications links, for example between a satellite and one or a plurality of earth stations, or between a radio base station of a terrestrial wireless communications system, transmitted signals become corrupted by noise due to a variety of factors, for example background noise, noise introduced through transmitter and receiver components, noise introduced through atmospheric transmission conditions, and interference introduced from other transmitters operating interfering carrier frequencies. In each case, for overlapping, but slightly different reasons, there is an advantage in being able to deal with a signal having as low a carrier to interference ratio, or as low a signal to noise ratio as possible. Toleration of a lower C/I or SNR ratio enables, use of lower power transmitter and receiver equipment, thereby in a satellite system, reducing the weight of equipment which needs to be launched, and reducing the power supply requirements of the equipment. In the case of terrestrial wireless systems, tolerating a lower carrier to interference ratio and signal to noise ratio enables mobile handsets having lower power transmitter apparatus, thereby reducing the size and power requirements of the handset, and possibly increasing the capacity of the overall terrestrial wireless system.
In order to recover transmitted signals which are received with a relatively low level of carrier to interference ratio and/or signal to noise ratio, it is known, prior to transmitting the signals to encode the signals with redundant bits of information according to known encoding algorithms. On receipt of the coded signals, known decoders are able to reconstruct parts of a signal which have been irretrievably corrupted due to noise or interference, and reconstruct the original signal from the redundant information contained in the coding. Such systems are known as forward error correction coded systems. Although the forward error correction codes add redundant bits to a signal to be transmitted, and effectively decrease the bandwidth available for data transmission, benefits are achieved in being able to decode signals which would otherwise be unable to be decoded, and improving the range, power consumption, and weight of transmitter and receiver equipment which can be used. The coding overhead of transmitting extra coding bits using forward error correction systems enable improvements in the effective bit error rate (BER) of a transmission link. In particular for satellite systems, the bit error rate of a satellite link is a limiting factor in performance of a satellite, since it has a direct impact on the power requirements of the launched transmitter/receiver equipment, and hence on the is cost of communications equipment. In the case of mobile wireless systems, the bit error rate of a link has a direct impact on the size and power requirements of the handsets which can be used.
A known forward error correction system comprises a convolutional coder and a Viterbi decoder, as is well known in the art, such as are available from Qualcom Incorporated. More recently, there have emerged in the prior art, a set of concatenated recursive codes known as "turbo codes", which may be used for the same purpose as Viterbi forward error correction codes, i.e. improving the bit error rate of transmission links, but which have improved performance compared with Viterbi coded systems. Turbo codes offer an approximate 3 dB improvement over Viterbi codes, which has the practical implication of allowing an approximate halving of transmitter power for a transmission link having a same bit error rate, as compared with a Viterbi coded link. Parallel concatenated systematic recursive codes (otherwise called "turbo codes") are described in "Near Shannon Limit Error-Correcting and Decoding: Turbo Codes (1)" by C Berrou, proceedings ICC May 1993. However, a problem with turbo code decoders compared with prior art forward error correction decoders is that turbo code decoders require increased data processing power. Typically, a known turbo code requires ten times as much processing power as a known Viterbi forward error correction decoder. Processing powers of the order of Giga instructions per second may be required. Digital signal processing apparatus having an ability to handle these volumes of instructions may comprise commercially available digital signal processing chip sets, or alternatively custom made decoder chip sets. Thus, the use of turbo codes instead of Viterbi codes, whilst potentially improving the transmitter/receiver equipment with respect to required transmission power and size of equipment, incurs the penalty of requiring around ten times as much signal processing power as Viterbi decoders. In the case of satellite systems, in addition to the increased cost of the extra signal processing power, the increased power consumption requirement is of concern. In the case of terrestrial wireless base stations, which operate in a cost competitive market, there is the disadvantage of the increased cost of the extra DSP chip sets required for providing the required signal processing power.